Use of sintered mixed carbonated for the confinement of radioactive carbon

ABSTRACT

The present invention relates to the use of a mixed carbonate of formula AB(CO 3 ) 2 , in which A and B are different and chosen from alkali metals, alkaline-earth metals and rare earths, for the containment of radioactive carbon.  
     This use may for example involve a process comprising: mixing C0 2  having a radioactive carbon to be contained, or a simple carbonate of an alkali, alkaline-earth or rare-earth metal having a radioactive carbon to be contained, with an aqueous solution of a mixture of ACl n  and BCl m  or with an aqueous solution of a mixture of A(OH) n  and B(OH) m  in order to obtain a precipitate of AB(CO 3 ) 2 , where n and m are integers sufficient to compensate for the charge of A and B respectively; recovery of the AB(CO 3 ) 2  precipitate in powder form; and then pressing and sintering of the powder at a temperature below the decarbonation temperature of the mixed carbonate manufactured in order to obtain sintered pellets of mixed carbonates for the containment of the radioactive carbon.

DESCRIPTION

1. TECHNICAL FIELD

The present invention relates to the use of sintered mixed carbonatesfor the confinement of radioactive carbon and to a radioactive carboncontainment process using these mixed carbonates.

Radioactive carbon, in ¹³C and essentially ¹⁴C form, is generated duringthe irradiation of fuels and is discharged in gaseous form (CO or CO₂)during the various steps in the reprocessing of spent fuels. The gaseousdischarge may represent 30% of the overall radiological impact of aradioactive waste reprocessing site on the environment.

There are several methods of trapping the carbon present in the gases,all resulting in the formation of simple carbonates of the BaCO₃, CaCO₃,SrCO₃ or MgCO₃ type. The present invention uses these carbonates, whichare radioactive via their carbon.

Because of its long half-life (5740 years), the contamination of theenvironment by ¹⁴C lasts for many years. It is therefore necessary tohave effective means for the containment of this carbon.

2. PRIOR ART

At the present time, only two types of matrix have been used hithertofor containing the carbon-bitumen matrices and cement matrices.

Bitumen matrices have been used for encapsulating carbonate effluents ofthe sodium carbon type in the case of the effluent processing from theperiod 1966-1971. This is therefore a proven technology. As regards theprocess, the safety of the bitumen-encapsulated carbonates cannot bequestioned, owing to the absence of any exothermic reaction between thesalt and the matrix. Although the maximum amount of carbonateincorporation into the bitumen has not generated specific tests, it isconceivable that this amount is close to that of bitumen encapsulantsfor radioactive sludge, i.e. about 45% by weight of the bitumenencapsulant.

However, bitumen encapsulation has many drawbacks. This is becausebitumen has a low stability to irradiation, the mechanical integrity ofbitumens is very poor because of its high creep, and the volume of wastegenerated by this matrix is very high, around 14 liters for 1 kg ofcarbon to be contained. Furthermore, this encapsulated material isfire-sensitive (inflammability risks), which poses a major problem inthe storage of radioactive waste.

At the present time, it is general practice to use a cement matrix asmatrix for the containment of carbon for carbonate encapsulation. Themain advantage of a cement matrix is that it has the benefit ofexperiment feedback from Sellafield and from specific studies regardingthe behavior of carbonates in this matrix.

However, the main drawback of this type of cement matrix is its inferiorchemical durability. It has been applied in particular to the case ofwaste intended for a surface storage center of the type of that of ANDRA(National Agency for the Management of Radioactive Waste) in theDepartement of Aube.

Furthermore, in the case of large quantities to be contained, thevolumes involved will be very large. The volume of waste generated bythis matrix is in fact around 12 liters for 1 kg of carbon to becontained.

From the results currently available for this type of matrix, it seemsthat containment would be possible in calcium carbonate form in cementsgenerally with a degree of encapsulation of between 30 and 35% byweight.

In the future it is envisioned to use fuels of the nitride or carbidetype that will probably be encapsulated with SiC. The amount of carbonto be contained, which may be a mixture of ¹²C and ¹³C, will thereforebe greater.

Owing to the aforementioned drawbacks of the prior art, and the newfuels that could be used in the future, it is therefore necessary topropose containment matrices of greater efficiency in terms of volume ofwaste created and also if possible in terms of chemical durability.

SUMMARY OF THE INVENTION

The object of the present invention is specifically to provide asolution to the many aforementioned drawbacks of the prior art byproposing novel containment matrices that are more efficient in terms ofvolume of waste created and also in terms of chemical durability. Theinvention also makes it possible to reduce the volume of waste by atleast a factor of four, and provides synthesis methods for the purposeof producing these matrices.

The present invention relates to the use of a mixed carbonate of formulaAB(CO₃) _((n+m)/2), the sintering temperature of which is below thedecarbonation temperature of the mixed carbonate and the hardness ofwhich is greater than or equal to 4 on the Mohs scale, in which A and Bare different and chosen from alkali metals, alkaline-earth metals andrare earths, and in which n and m are positive integers such that thecharge of AB(CO₃) _((n+m)/2) is neutral, for the containment ofradioactive carbon.

The present invention also relates to a radioactive carbon containmentprocess, comprising the following steps:

-   -   a) mixing CO₂ having a radioactive carbon to be contained, or a        simple carbonate of an alkali, alkaline-earth or rare-earth        metal having a radioactive carbon to be contained, with an        aqueous solution of a mixture of ACl_(n) and BCl_(m) or with an        aqueous solution of a mixture of A(OH)_(n) and B(OH)_(m) in        order to obtain a precipitate of AB (CO₃) _((n+m)/2) where A and        B are different and chosen from alkali metals, alkaline-earth        metals and rare earths, and n and m are positive integers such        that the charge of ACl_(n), BCl_(m), A(OH)_(n), B(OH)_(m) and AB        (CO₃) _((n+m)/2) is neutral;    -   b) recovering the AB (CO₃)₂ precipitate obtained in step a) in        powder form;    -   c) optionally rinsing said powder; and    -   d) pressing the powder and sintering it at a sintering        temperature below the decarbonation temperature of the        synthesized mixed carbonate in order to obtain sintered pellets        of mixed carbonates of formula AB (CO₃) _((n+m)/2,) the hardness        of which is greater than or equal to 4 on the Mohs scale, and        containing the radioactive carbon.

According to the invention, A and B may advantageously be chosen fromNa, K, Ca, Ba, Mg and Sr. This is because these elements are easilyavailable and are of low cost.

For the containment of the radioactive carbon in the form of CO₂ presentin gaseous effluents, for example emanating from irradiated nuclear fuelreprocessing plants, there are various trapping processes. The mostcommonly employed processes are the following: double alkali process;direct hydroxide reaction process; and gas/solid process. Theseprocesses are known to those skilled in the art.

Briefly:

-   -   1) in the double alkali process, the CO₂ is firstly trapped in        sodium carbonate form in a packing column sprayed with for        example 4 N sodium hydroxide. This sodium carbonate then reacts        in a reactor with calcium hydroxide in order to form calcium        carbonate, which is the chemical form useful in the process of        the invention for storing carbon-14. The trapping of the CO₂        takes place according to the following reactions:        2NaOH+CO ₂ →Na ₂ CO ₃₊ H ₂ O        Na ₂ CO ₃₊ Ca(OH)₂→2NaOH+CaCO ₃.

In the first step, it is possible to replace NaOH with KOH. In theaforementioned example, the solution emanating from the column is thatof about 1N sodium hydroxide and 3.2 M Na₂CO₃. This solution then reactswith Ca(OH)₂ to form the insoluble calcium carbonate and to regeneratethe 4N sodium hydroxide. The solution is then filtered to recover thecalcium carbonate, which is preferably washed to remove the residualsodium hydroxide;

-   -   2) in the direct hydroxide reaction process, the CO₂ reacts        directly with a hydroxide according to the reaction:        ${{\frac{2}{n}M\quad({OH})_{n}} + {CO}_{2}}->{{M_{\frac{2}{n}}{CO}_{3}} + {H_{2}O}}$        M being chosen from alkali metals, alkaline-earth metals and        rare earths and n being a positive integer such that the charge        of M(OH)_(n) and of M_(2/n)CO₃ is neutral. M is for example        chosen from Na, K, Ca, Ba, Mg and Sr. For example NaOH, Ba(OH)₂,        Ca(OH)₂ and Mg(OH)₂;

3) in the gas/solid process, the chemical reaction used is the same asthat for the process using an aqueous suspension. Only the techniquewhereby the reactants are brought into contact with each other isdifferent, since for this process the gas is brought directly intocontact with the solid reactant. The trapping takes place according tothe reaction:M(OH)₂ +CO ₂ →MCO ₃ +H ₂ Oin which M is as defined above. The ¹⁴CO₂ is thus trapped directly in asolid. With barium hydroxide for example, trials have been carried outin a fixed bed and in a fluidized bed. Among the barium hydroxidestested, the most reactive with respect to CO₂ is the octahydrateBa(OH)_(2·)8H₂O. The reaction is as follows:Ba(OH)₂·8H ₂ O+CO ₂ →BaCO ₃+9H ₂ O.

This process has the advantage over a gas/liquid process of notrequiring a liquid/solid separation.

The benefit of these processes 1), 2) and 3) is that the radioactivecarbon is trapped in the form of simple carbonates, for example of theBaCO₃, CaCO₃, SrCO₃ or MgCO₃ type, which can be directly used in thepresent invention.

In addition, according to the invention, the simple alkali,alkaline-earth or rare-earth metal carbonate, the radioactive carbon ofwhich is to be contained, may be obtained by trapping the radioactivecarbon, in CO₂ form, from a gaseous effluent, said trapping beingadvantageously chosen from a double alkali process, a direct hydroxidereaction process and a gas/solid process.

According to the invention, a first method of implementing the processof the invention in order to manufacture sintered mixed carbonates ofAB(CO₃)₂ type may consist in step a) of the process in making Na₂CO₃,for example obtained by one of the aforementioned processes, dissolvedin water, react at room temperature with an aqueous solution ofACl_(n)+BCl_(n), for example CaCl₂+BaCl₂ dissolved in water, instoichiometric molar proportions. These proportions are for example: 2mol of Na₂CO₃₊1 mol of CaCl₂₊1 mol of BaCl₂ give 1 mol of BaCa(CO₃)₂₊4mol of NaCl. The reaction is instantaneous and results in the formationof the mixed carbonate, which precipitates, and dissolved NaCl.

According to the invention, a second method of implementing the processof the invention in order to manufacture sintered mixed carbonates ofAB(CO₃)₂ type may consist in making Na₂CO_(3,) obtained for example byone of the aforementioned processes, dissolved in water, react with anaqueous solution of A(OH)_(n)+B(OH)_(n), for example Ca(OH)₂+Ba(OH)₂dissolved in water, in stoichoimetric molar proportions. Theseproportions are for example: 2 mol of Na₂CO₃₊1 mol of Ca(OH)₂₊1 mol ofBa(OH)₂ give 1 mol of BaCa(CO₃)₂₊2 mol of NaOH.

According to the invention, a third method of implementing the processof the invention in order to manufacture sintered mixed carbonates ofAB(CO₃)₂ type may consist in making the CO₂ whose radioactive carbon isto be contained react directly with a mixture of hydroxidesA(OH)_(n)+B(OH)_(n), with A and B as defined above, in order to form themixed carbonate. This reaction may be carried out for example by agas/solid process as described above (process 3) for trapping thegaseous CO₂.

The next step b) of the process of the invention may consist for examplein carrying out a solid/liquid separation, for example by simplefiltration, so as to recover the mixed carbonate in powder form.

The powder obtained may be rinsed in step c). This rinsing is verypreferably carried out with ultrapure distilled water.

The pressing and the sintering may be carried out at any sinteringpressure and temperature and for any sintering time suitable forobtaining a sintered mixed carbonate, provided that the temperature isbelow the decarbonation temperature of the mixed carbonate synthesized.This is because, below 500° C., no sintering is observed, or theduration of the treatment is too long. Above 680° C., a decarbonationeffect is observed, which opposes the expected containment.

According to the invention, for example in the case of BaCa(CO₃)₂, thepressing may be advantageously carried out at a pressure ranging from 10to 20 MPa and the sintering may be advantageously carried out at atemperature ranging from 500° C. to a temperature below 680° C. for 1 to3 hours. Preferably, the pressing may be carried out at a pressure of 14to 16 MPa, and the sintering at a temperature of 550 to 600° C. for 1hour 45 minutes to 2 hours 30 minutes. More preferably still, thepressing may be carried out at a pressure of 15 MPa and the sintering ata temperature of 580° C. for 2 hours.

In this example, by pressing under the aforementioned conditions of theprocess of the invention it is possible to obtain pellets advantageouslyhaving a densification of greater than 90%, a high hardness, between 4and 4.5 on the Mohs scale, namely a hardness between fluorite andapatite, and a carbon content between 7 and 10% by weight for a densityof 3.7, which means a volume of 3.3 liters of waste for containing 1 kgof carbon.

The process of the invention allows the radioactive carbon to becontained directly in a sintered carbonate without encapsulation. Themixed carbonates of the present invention advantageously have thefollowing properties:

-   -   high decarbonation temperatures, greater than 300° C., in order        to meet the criteria defined for storing radioactive waste;    -   they are not soluble in water, which prevents leaching effects;    -   they have a high hardness, greater than or equal to 4; and    -   they have sintering temperatures below the decarbonation        temperature of the mixed carbonate synthesized.

The volume of waste generated by a sintered carbonate according to thepresent invention is around 3 liters for 1 kg of carbon to be contained,depending on the carbonate used. This volume is substantially smallerthan those obtained with the processes of the prior art.

Other characteristics and advantages will become apparent on reading thefollowing examples given by way of illustration, with reference to theappended drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an X-ray spectrum (intensity (I) (counts) (in a.u.) as afunction of the diffraction angle (20θ) of an alstonite ceramic obtainedaccording to the present invention.

FIG. 2 is a DTA/TGA spectrum (dilatometric analyzer) showing that thedecarbonation of a BaCa(CO₃)₂ powder starts at 680° C. Plotted on theleft of this figure, on the y-axis, is the heat flux (F) in μV, and onthe right the weight loss (ΔW) in μg. Curve 1 shows the differentialthermal analysis (DTA) (heat flux), curve 2 shows the thermogravimetricanalysis (TGA) (weight loss) and curve 3 shows the interpretation of theweight loss.

FIG. 3 is an image of a material according to the invention obtained byscanning electron microscopy. The magnification scale is indicated onthe photograph.

EXAMPLES

Example 1: case of a mixed BaCa(CO₃)₂ carbonate

-   -   21.198 g of Na₂CO₃ were dissolved in 1 liter of water in beaker        1;    -   48.85 g of BaCl₂₊22.196 g of CaCl₂ were dissolved in 2 liters of        water in beaker 2.

The contents of the two beakers were then mixed. A precipitate wasobtained.

The precipitate obtained was filtered and then rinsed three times withultrapure distilled water.

The powder obtained was the desired mixed carbonate, namely BaCa (CO₃)₂.

The decarbonation of this BaCa (CO₃)₂ powder advantageously started at680° C., as the DTA/TGA spectrum illustrated in the appended FIG. 2shows.

By pressing at 15 MPa followed by natural sintering at 580° C. for 2hours, it was possible to obtain pellets having the followingproperties:

-   -   a densification of greater than 90% (see FIG. 3);    -   a high hardness, of between 4 and 4.5 on the Mohs scale;    -   a carbon content of around 8% by weight for a density of 3.7,        which means a volume of 3.31 of waste for containment of 1 kg of        carbon; and    -   a pKs of 8.6 at 90° C. for the reaction:        Ba _(1/2) Ca _(1/2)(CO ₃)        1/2 Ba ²⁺ + 1/2 Ca ²⁺ CO ₃ ²⁻.

These pellets were examined under a scanning electron microscope. FIG. 3is a photograph of this examination. By synthesizing the BaCa(CO₃)₂carbonate it is possible to obtain an alstonite ceramic having a fewBaCO₃ impurities, as the X-ray (XRD) spectrum of FIG. 1 and thephotograph obtained in scanning electron microscopy of FIG. 3 show. Thisceramic, which has a much higher hardness than that of the simplecarbonates BaCO₃ and CaCO_(3,) is obtained by natural sintering. Thus,the nonfriable material obtained can be easily handled.

1. The use of a mixed carbonate of formula AB(CO ₃) _((n+m)/2), thesintering temperature of which is below the decarbonation temperature ofthe mixed carbonate and the hardness of which is greater than or equalto 4 on the Mohs scale, in which A and B are different and chosen fromalkali metals, alkaline-earth metals and rare earths, and in which n andm are positive integers such that the charge of AB(CO₃) _((n+m)/2) isneutral, for the containment of radioactive carbon.
 2. The use asclaimed in claim 1, in which A and B are different and chosen from Na,K, Ca, Ba, Mg and Sr.
 3. The use as claimed in claim 1, in which themixed carbonate is chosen from BaCa(CO₃)₂.
 4. The use as claimed inclaim 1, in which the mixed carbonate is sintered for the containment ofthe radioactive carbon.
 5. The use as claimed in claim 1, in which theradioactive carbon comes from a gaseous effluent of an irradiatednuclear fuel reprocessing plant.
 6. A radioactive carbon containmentprocess, comprising the following steps: a) mixing CO₂ having aradioactive carbon to be contained, or a simple carbonate of an alkali,alkaline-earth or rare-earth metal having a radioactive carbon to becontained, with an aqueous solution of a mixture of ACl_(n) and BCl_(m)or with an aqueous solution of a mixture of A(OH)_(n) and B(OH)_(m) inorder to obtain a precipitate of AB (CO₃) _((n+m)/2) where A and B aredifferent and chosen from alkali metals, alkaline-earth metals and rareearths, and n and m are positive integers such that the charge ofACl_(n), BCl_(m), A(OH)_(n) and B(OH)_(m) is neutral; b) recovering theAB(CO₃)₂ precipitate obtained in step a) in powder form; c) optionallyrinsing said powder; and d) pressing the powder and sintering it at asintering temperature below the decarbonation temperature of thesynthesized mixed carbonate in order to obtain sintered pellets of mixedcarbonates of formula AB (CO₃) _((n+m)/2), the hardness of which isgreater than or equal to 4 on the Mohs scale, and containing theradioactive carbon.
 7. The process as claimed in claim 6, in which A andB are different and chosen from Na, K, Ca, Ba, Mg and Sr.
 8. The processas claimed in claim 6, in which the mixed carbonate is chosen fromBaCa(CO₃)₂.
 9. The process as claimed in claim 6, in which the pressingis carried out at a pressure ranging from 10 to 20 MPa, and thesintering at said temperature for 1 to 3 hours.
 10. The process asclaimed in claim 6, in which the pressing is carried out at a pressureof 14 to 16 MPa, and the sintering at a temperature of 550° C. to 600°C. for 1 hour 45 minutes to 2 hours 30 minutes.
 11. The process asclaimed in claim 6, in which the simple alkali, alkaline-earth orrare-earth metal carbonate, the radioactive carbon of which is to becontained, is obtained by trapping the radioactive carbon, in CO₂ form,in accordance with a process chosen from a double alkali process, adirect hydroxide reaction process and a gas/solid process.
 12. Theprocess as claimed in claim 6, in which the CO₂ having a radioactivecarbon to be contained, or a simple carbonate of an alkali,alkaline-earth or rare-earth metal having a radioactive carbon to becontained, comes from an effluent of an irradiated nuclear fuelreprocessing plant.